This invention relates to control systems of hydraulic construction machinery, and more particularly it is concerned with a control system of hydraulic construction machinery provided with at least one variable-displacement hydraulic pump driven by a prime mover for driving desired working elements through actuators for operation.
One typical example of the hydraulic construction machinery is a wheel type hydraulic excavator which generally comprises a prime mover or an engine, means for controlling the number of revolutions of the engine such as a fuel injection device including an engine lever operated by an operator in his cab and a governor lever connected to the engine lever, at least one variable-displacement hydraulic pump driven by the engine, means for controlling the displacement volume of the hydraulic pump, a plurality of actuators driven by pressurized fluid discharged by the hydraulic pump and a plurality of working elements driven by the respective actuators. The working elements generally comprise left and right travel devices and other working elements including a swing and front attachments, such as a boom, an arm and a bucket.
In this type of hydraulic excavator, operation levers are provided one for each working element and operated by the operator to actuate a directional control valve located between the hydraulic pump and the corresponding actuator, so that the pressurized fluid discharged by the hydraulic pump driven by the engine can be fed to the corresponding actuator to drive the same to operate the working element. Thus, the pressurized fluid supplied to the actuators moves either one or both of the travel devices, causes the swing to move in swinging movement and renders the front attachments operative to perform excavation.
In a conventional hydraulic excavator in general, the maximum value of the number of revolutions of the engine controlled by a fuel injection device is constant, and the maximum value of the displacement volume of the hydraulic pump controlled by the regulator or the maximum value of the tilting angle of a swash plate when the hydraulic pump is of a swash plate type is also constant. The maximum constant number of revolutions and the maximum constant displacement volume are uniquely set such that when the pump is in a condition for the maximum displacement volume, the hydraulic pump provides a desired maximum delivery at the maximum number of revolutions and at the same time a desired pump consumption horsepower can be obtained at the maximum number of revolutions. In other words, in this type of hydraulic excavator, the maximum delivery of the pump and the engine horsepower characteristics are uniquely decided by the product of the maximum number of revolutions and the maximum displacement volume and by the set maximum number of revolutions, respectively.
Meanwhile the working elements of a hydraulic excavator each do work of a content which differs from one working element to another, such as traveling, swinging, excavation, etc. Although each working element does work of the predetermined content, loads applied to working elements differ from each other depending on the amount of work. For example, in the case of travel devices, a load applied to them is low when the road surface is flat and it is high when the travel is uphill. In each front attachment, for example, there are heavy-excavation requiring a great amount of work and light-excavation involving a relatively small amount of work.
Thus, in the conventional hydraulic excavator, a problem arises when it is necessary to do work in which the load varies greatly, because of the fact that the maximum number of revolutions of the engine and the maximum displacement volume of the hydraulic pump are set at a constant in value as described hereinabove.
For example, wheel type hydraulic excavators are allowed to travel on roads in addition to moving around at the site of the work, and their maximum speeds are set at relatively high levels in many cases. In Japan, for example, their maximum speed of travel is set at 35 km/h. This makes it necessary for a wheel type hydraulic excavator to be able to travel at the maximum speed of 35 km/h. Roads include flat ones and slopes. Thus it is desirable that the hydraulic excavator be able to travel at the legally set maximum speed of 35 km/h regardless of the type of road on which it travels.
Because of this, a wheel type hydraulic excavator is faced with specific problems that should be obviated, because it has a higher speed of travel than a crawler type hydraulic excavator and it preferably is able to travel at the legally set maximum speed of 35 km/h even when it travels uphill under a high load.
To be more specific, if the maximum number of revolutions of the engine and the maximum displacement volume of the hydraulic pump are set such that, by placing emphasis on the ability to travel uphill under a high load, the engine has a horsepower characteristic which will enable a high pump consumption horsepower to be obtained and will provide a high delivery by the pump, the hydraulic excavator will suffer the disadvantages of being high in fuel consumption and noise level when it does a type of work, such as travelling on a flat load or doing excavation under a low load, which requires a lower pump consumption horsepower than the uphill travel. Conversely, if the above values are set by placing emphasis on the ability to travel on flat roads or perform excavation, it will be impossible to obtain a high pump consumption horsepower, so that no sufficiently high horsepower can be obtained when travelling uphill and it will be impossible to travel at the legally set maximum speed of 35 km/h. Considering the excavation work done by a front attachment; if, by placing emphasis on performing heavy-excavation, the maximum number of revolutions of the engine and the maximum displacement volume of the hydraulic pump are set such that the engine has a horsepower characteristic enabling a high pump consumption horsepower and a maximum delivery by the hydraulic pump becomes a suitable value, it will be possible to perform heavy-excavation at a desired speed but part of the engine horsepower will be wasted when light-excavation is performed at the set values. Thus the hydraulic excavator will suffer the disadvantages that its fuel consumption is high and the engine produces unnecessary high noises. Conversely, if the maximum values are set by placing emphasis on doing light-excavation, it will be impossible to perform heavy-excavation satisfactorily. This problem is shared by a wheel type hydraulic excavator and a crawler type hydraulic excavator.
To sum up, if the maximum number of revolutions of the engine and the maximum displacement volume of the hydraulic pump are set in construction machinery, such as a hydraulic excavator, by placing emphasis on its ability to perform operations under a high load, then the problem that its fuel consumption, noise level and costs are high will arise when it operates under a low load. If the maximum values are set by placing emphasis on its ability to do work under a low load, it will be impossible to achieve a high performance in performing operations under a high load.
JP-A-No. 58-135341 (Japanese patent application No. 16349/82) laid open to public inspection on Aug. 11, 1983 in Japan discloses a control system of hydraulic construction machinery which is provided with means for sensing the operating conditions of the actuators and which is operative to increase the number of revolutions of the engine and decrease the displacement volume of the hydraulic pump when it is sensed that an actuator tending to have a high load applied thereto is in operation and which is operative to decrease the number of revolutions of the engine and increase the displacement volume of the hydraulic pump when it is sensed that an actuator tending to have a low load applied thereto is in operation, to thereby avoid a sudden change in the delivery by the pump with respect to a fluctuation in load to save energy and improve the operability of the machinery.
As described hereinabove, loads applied to a working element may vary one from another depending on the amount of work. The control system referred to hereinabove can not cope with such variation in load. That is, the problem encountered when the hydraulic excavator travels on flat roads with the maximum values having been set by placing emphasis on the ability to travel uphill at high speed and the problem raised when light-excavation is performed with the maximum values having been set by placing emphasis on the ability to perform heavy-excavation, as described hereinabove, have not been dissolved by the control system referred to hereinabove. In other words, the control system referred to hereinabove is unable to obviate the fundamental disadvantage of conventional hydraulic excavators when work done by a working element shows a wide range of variations in amount.
The inventors of the subject application have proposed to use a control system of hydraulic construction machinery which aims at dissolving the aforesaid problems as disclosed in Japanese patent application No. 239897/85 of which corresponding applications are U.S. Ser. No. 904118, Korea Appln. No. 7274/86, India Appln. No. 670/Cal/86, China Appln. No. 86106816 and EPC Appln. No. 86112330.5.
The control system comprises a prime mover, revolution number control means for controlling the number of revolutions of the prime mover, at least one variable-displacement hydraulic pump, driven by the prime mover displacement volume control means for controlling the displacement volume of the hydraulic pump, at least one actuator driven by pressurized fluid delivered by the hydraulic pump, maximum revolution number altering means associated with the revolution number control means for altering the maximum number of revolutions of the prime mover between at least a first maximum value and a second maximum value greater than the first maximum value, maximum displacement volume altering means associated with the displacement volume control means for altering the maximum displacement volume of the hydraulic pump between at least a first maximum value and a second maximum value greater the first maximum value and information provider means for providing information on the operation mode of the actuator, whereby the maximum revolution number altering means and maximum displacement volume altering means are controlled by an output signal of the information provider means to provide a maximum number of revolutions and a maximum displacement volume which suit the operation mode indicated by the output signal. More specifically, maximum revolution number limiter means limiting the maximum number of revolutions to either the first maximum value or the second maximum value serves as the maximum revolution number altering means, and a mode selection switch capable of selecting either a power mode or an economy mode serves as the information provider means. The maximum revolution number altering means and maximum displacement volume altering means are controlled in such a manner that, when the power mode is selected by the mode selection switch, a combination of the second maximum value of the number of revolutions and the first maximum value of displacement volume suiting a high load operation is selected, and, when the economy mode is selected, a combination of the first maximum value of the number of revolutions and the second maximum value of displacement volume suiting a low load operation is selected. This enables a satisfactory performance to be obtained in both the high-load and low-load operations, and makes it possible to dissolve the problems encountered with regard to a high noise level and a high fuel consumption in the low load operation. The maximum values are set such that the product of the first maximum value of the number of revolutions and the second maximum value of displacement volume is essentially equal to the product of the second maximum value of the number of revolutions and the first maximum value of displacement volume. Thus, even if the aforesaid combinations are changed when the number of revolutions of the prime mover the has either the first maximum value or the second maximum value declared by the maximum revolution number limiter means, the delivery by the hydraulic pump shows substantially no change.
The invention disclosed in Japanese patent application No. 239897/85 which uses the maximum revolution number altering means of a type which limits the maximum number of revolutions of the prime mover has raised the problem that a variation occurs in the delivery by the hydraulic pump if the mode selection switch is actuated to give a command to change the operation modes when no limit is placed on the maximum value of revolutions of the prime mover. That is, if the mode selection switch is actuated to alter the mode to the economy mode when the power mode has been selected by the mode election switch and the actual number of revolutions of the prime mover lies between the first maximum value and the second maximum value, the maximum displacement volume will be altered to the second maximum value and the maximum number of revolutions will be limited to the first maximum value. As described hereinabove, the product of the first maximum value of the number of revolutions and the second maximum value of displacement volume is set to be substantially equal to the product of the second maximum value of the number of revolutions and the first maximum value of displacement volume, but does not become equal to the product of a number of revolutions smaller than the second maximum value of the number of revolutions and the first maximum value of displacement volume. Thus a variation will be caused to occur in the quantity of pressurized fluid delivered by the hydraulic pump. Also, if the mode selection switch is actuated to give a command to change the operation modes when the number of revolutions of the prime mover is smaller than the first maximum value, only the maximum displacement volume will be altered and the number of revolutions of the prime mover will not be altered, no matter which mode is to be selected. This will also cause a change to occur in the quantity of pressurized fluid delivered by the hydraulic pump.